Planar optical arrangements giving rise to a synthetic, more or less three-dimensional, image or an image that changes its appearance at different angles have been used in many applications. Besides purely esthetical uses, such arrangements have been used e.g. as security labels on bank-notes or other valuable documents, identification documents etc. The synthetic three-dimensional images have also been used for providing better geometrical understanding of complex shapes in e.g. two-dimensional information documents.
In the published international patent application WO 94/27254 [1], a security device is disclosed. The security device comprises an array of microimages which, when viewed through a corresponding array of substantially spherical microlenses, generates a magnified image. This result is achieved according to the long known Moiré effect and was now applied to provide security labels with images having a three-dimensional appearance. The array may also be bonded to the array of microimages.
In the published US patent application US 2005/0180020 [2], a micro-optic security and image presentation system is disclosed, being based on a similar basic idea. A film material utilizes a regular two-dimensional array of non-cylindrical lenses to enlarge micro-images or image data bearer structures of an image plane. By adapting the focal properties of the lenses, the distance between the lenses and the image plane, the diameter of the lenses, different magnifications, field of view, apparent depth etc. may be changed.
In an ideal spherical microlens, there are still effects causing unsharpness of the created image. Spherical aberrations are present, which gives rays passing the microlens surface at a relative low angle a different focal point. Furthermore, even without spherical aberrations, blurring is introduced when the image is viewed in a non-perpendicular direction, since the focal length of the spherical microlens does not exactly match the distance between the point at which a paraxial ray passes the lens and the position of the image data bearer structure. This is in US2005/0180020 solved by providing image data bearers that have a certain non-negligible height. Even at slanted view directions, the image data bearer information will thereby be present at a focal point of the microlens. However, such arrangements do only partly solve the problem, since they instead introduce a blurring effect due to the very same extension in height. Rays that have their actual focal point besides a certain image data bearer structure will anyway be influenced by the structure on its way to or from the focal point, when the viewing angle is tilted enough. The approach in US2005/0180020 for solving the blurring problem therefore does not solve the problem entirely.
In the published international patent application WO 2007/115244 a sheeting presenting a composite floating image is disclosed. A layer of microlenses covers a surface with radiation sensitive material. By exposing the arrangement for high-energy radiation, the radiation sensitive material records the distribution of radiation that has passed through the lens array. The radiation distribution carries information about the three-dimensional properties of the radiation. When the arrangement later is exposed for light, a floating image resembling the high-energy radiation can be viewed. This arrangement is thus a variation of integral photography. However, the use of photographic recording without developing processes gives images of low quality and the need of radiation exposure of the assembled arrangement is unsuitable for low-cost industrial production.